THE PALEONTOLOGY OF MINNESOTA. 



[Archinacella vallda. 



by regular, concentric, sublamelliform striae.* The present species differs further 

 in wanting the obtuse carination of the dorsum and in having a smaller apex. 



Formation and locality. Black River group, Rhinidictyaand Ctenodonta beds, Minneapolis, St. Paul, 

 Cannon Falls. Chatfleld, and near Fountain, Minnesota. Of all the patelliform shells occurring in Minne- 

 sota rocks this is the only one that is reasonably abundant. We have seen about twenty specimens. 



Collections. Geological and Natural History Survey of Minnesota; Geological Department, University 

 of Minnesota; W. H. Scofleld; E. O. Ulrich. 



Museum Register, Nos. 4067 and 8723. 



ARCHINACELLA VALIDA Sardeson sp. 



PLATE LXI, FIGS. It and 15. 

 Tryblidium validum SAKDESON, 1892, Bull. Minn. Acad. Nat. Sci., vol. iii, p. 337. 



Shell rather small, oblique-subconical, strongly convex, the dorsum narrowly 

 rounded; outline elliptical, the width and length about as four is to five; aperture 

 not arched; apex just within the anterior margin, laterally compressed, but little 

 incurved in casts, the point about two-thirds of the bight of the shell above the 

 edge of the aperture; beneath the apex the outline is deeply cut out in a side 

 view. Surface markings consisting of rather obscure lines of growth. Length 

 (small specimen) 16.5 mm.; width 13.4 mm.; hight nearly 7 mm. In another 

 specimen, proportionally larger, the length reaches 20 mm. 



We believe the specimens here figured and described are specifically the same 

 as the one which Mr. Sardeson recently proposed to call Tryblidium validum. A 

 comparison of our figures with his, it is true, brings out some differences, the 

 outline of the aperture in ours being more regularly elliptical and the profile in 

 the side view less convex centrally. It would appear, however, that in Mr. Sarde- 

 son's figure 2 the convexity is exaggerated, since in his description he gives the 

 hight of the shell as equalling only half the width. And this is true of both the 

 specimens here referred to his species. In this same figure 2 he shows three 

 detached muscle scars, of so large a size that the whole ring would be made up of 

 only three or possibly four pairs! But this would be so much out of the regular 

 order that we are fully convinced that they rest on faulty observation. So far as 

 our specimens are concerned nothing of the kind is to be observed. On the contrary 

 we notice faint indications of the Archinacella band, and hence refer the species to 

 this genus. 



Specifically A. valida is nearer A. (Carinaropsis) patelliformis Hall, from the 

 Trenton of New York,than any other form known to us. The lateral profile especially 

 is nearly or quite the same in the two species. The New York species, however, is 

 smaller, has more distinct surface marking's and is obtusely carinated on the back. 



In comparing this and other apeclus with A. j>alellifiiniiin. we refer to the Trenton form only, and, as wo have not seen 

 I hr original types, we are obliged to depend solely on Hull's azures and description for our conception of their chara<-ti-i>. 



